Pollen Assemblages Of The Human Disturbed Vegetation In Arid And Semi-arid Northwestern China And Its Preliminary Application

The surface pollen analysis of the previous studies has contributed to an important objective reconstruction of the paleo-vegetation and paleo-climate. However, they mainly focus on these areas where there are hardly affected by the human activities, while both the natural factors and the human activities change the vegetation and environment. Therefore, the human planted and human disturbed vegetation should be put more emphasis in order to extract the ecological information of human activities, which can be not only better for us to restore the paleo-climate and paleo-vegetation. but also might improve the prediction of the future climate and environment change. The arid and semi-arid northwestern China, with strong response to global climate fluctuations, has always been a hotpot of global change research, where human activities have relatively small impact on terrestrial ecosystem. It shows complex geomorphic mosaic landscape of oasis and Gobi desert, including the temperate desert zone and oases zone. The pollen assemblages characters of the nature vegetation, human disturbed vegetation and human planted can be tested. Furthermore, the different human activities intensity impacted on the vegetation and environment are also discussed.A total of 73 modern soil samples were collected in arid and semi-arid areas. Meanwhile,27 wetlands samples were collected in the Linze wetland, from which the early human impact intensities and land use conditions can be investigated, as well as some new evidence for studying human activity.The results of the surface pollen analysis show that:the appearance of the crops pollen might be an important indicator of revealing the human activities and the crops pollen content combined with the pollen assemblages may be reflect the human impact intensities.The crops pollen is the main component but not dominant in the pollen assemblages of the area where the human activities are more serious, the pollen characters of which also reflect the regional vegetation conditions around the farmlands. While in the transitional zone between the farmland zone and natural vegetation, where there are relatively less severe human activities, whose pollen assemblages are dominated by weed pollen types, with more shrubs pollen types and fewer crops pollen. Besides, the pollen assemblages of the alpine meadow are consistent with the local vegetation, in which Cyperaceae and Graminae are the major pollen types, but the dry herbs pollen content is low (mean11.1%) with Chenopodiaceae is the main pollen type. And the pollen character of the Gobi desert where there are rare human activities, are predominant with Chenopodiaceae. Ephedra (up to 78%) and Artemisia (median 11.3%). the pollen types are also fewer. The pollen assemblages and the crops pollen content of the same types of farmland located in different regions or altitude are also different, which may be related to the local vegetation condition, pollen productivity of the crops and the methods of their farming. In these sampling sites such as farmlands, where human impact intensities are stronger and the humidity is higher, whose vegetation coverage is up to 100%, the pollen concentration are lower (mean 7314 grains/g). This phenomenon may be related to several factors, such as the pollen preservation conditions affected by the fertilizers added by the farmers, and lower pollen percentage of the weeds pollen due to the weeding of human, as well as lower pollen productivity and special pollination of some crops pollen types. While in the wildlands. there are less servere human activities and the pollen concentration is higher, which suggest that the pollen type diversity might be correlated with the pollen concentration. In the alpine meadow, vegetation and pollen concentration are very high. In the Gobi desert, the vegetation coverage of some sampling samples are lower but the pollen concentration are relatively higher, which is probably due to the location and time of the sampling sites, as well as the high pollen productivity of some species such as Chenopodiaceae and Ephedra. Principal component analysis (PCA) can distinguish the sampling sites affected with different human impact intensities. The PCA diagram shows that the axis-1 may indicate that the humidity gradient is the main factor influencing the vegetation composition and plant growth, and the axis-2 might indicate that the temperature is the key. In all, we can conclude that the humidity and temperature are very essential for the plant growth and vegetation composition, and the humidity gradient is the most important factor that can reflect human impact intensities.The pollen assemblages of Linze wetlands indicate that between 1120a B.P. (about 830 AD, the end of Tang Dynasty) to 400 a B.P. (about 1550 AD, the mid-Ming Dynasty), the pollen assemblage is dominant by Graminae (<40μm) and Compositae, with some Chenopodiaceae and fewer Artemisia pollen but there is no crops pollen, which may show that it was a stable wetland meadow in this period, and there are rare agricultural activities. However, after 400a B.P. (about 1550 AD, the mid-Ming Dynasty), the arboreal pollen content decrease with the increasing of cereal-type Gramineae and the apparence of some Cannabaceae, in addition, the percentages of Chenopodiaceae, Artemisia, Compositae and natural Graminae pollen also increase, which might reflect that the agricultural activity of Hexi Corridor in this stage begin to develope again and the scale of the agricultural production become larger.